1. Field of the Invention
The subject of the present invention is a micro-electro-mechanical gyroscope having electrically insulated regions.
2. Description of the Related Art
As is known, integrated gyroscopes of semiconductor material, obtained through the MEMS (Micro-Electro-Mechanical Systems) technology, operate according to the theorem of relative accelerations, exploiting the Coriolis acceleration. In particular, when a linear velocity is applied to a mobile mass rotating at an angular velocity, in the reference system fixed to the mobile mass, the mass is subject to an apparent force, referred to as the Coriolis force, which brings about a displacement thereof perpendicularly to the linear velocity and to the axis of rotation. The apparent force can be thus detected by supporting the mobile mass via springs that enable a displacement thereof in the direction of the apparent force. According to Hooke's law, this displacement is proportional to the apparent force, which is correlated to the angular velocity. Consequently, detection of the displacement of the mobile mass enables detection of the Coriolis force and thus of the angular velocity.
In gyroscopes of the type considered, the displacement of the mobile mass is detected in a capacitive way, by measuring at resonance the capacitance variations caused by the movement of mobile sensing electrodes rigid with the mobile mass and interfaced or comb-fingered with fixed sensing electrodes.
An embodiment of an integrated gyroscope made as MEMS in planar technology is for example described in U.S. Pat. No. 6,766,689, assigned to STMicroelectronics Srl. This integrated gyroscope is formed by an actuation assembly, by a sensitive mass actuated in a first direction lying in the plane of the sensitive mass and moreover mobile in a second direction, which also lies in the plane of the sensitive mass, and by a capacitive sensing system facing the sensitive mass and capable of detecting the movements of the latter in the second direction. The gyroscope has a rotation axis perpendicular to the plane of the sensitive mass so that, subject to an external angular velocity, the sensitive mass moves in the second direction.
In another patent application filed in the name of the present applicant and assigned to STMicroelectronics Srl. (U.S. Pat. No. 6,928,872), a gyroscope is moreover described capable of detecting the Coriolis force even when this acts perpendicularly to the plane of the sensitive mass.
In U.S. application Ser. No. 11/177,474, filed Jul. 7, 2005 and assigned to STMicroelectronics Srl., for the purpose of obtaining high performance and preventing cross-talk between actuation and reading of the signal, the actuation assembly and the sensitive mass, at least in the part facing the capacitive sensing system, are biased at different voltages. In particular, given that the mechanical assemblies are formed in a same structural layer (epitaxial or pseudo-epitaxial layer), they are electrically decoupled via insulation regions closed at the top and at the bottom by plugs that guarantee on the one hand the desired electrical insulation and, on the other hand, protection of the insulation during the fabrication steps of the gyroscope, in particular during trench etching (Deep Reactive Ion-Etching or DRIE of the epitaxial silicon) for defining the structure and etching sacrificial layers so as to free it.
This solution is highly advantageous, but does not afford protection of the lateral portions of the insulation regions which face the delimitation trenches. Even though the lack of protection on this edge during the fabrication process is not essential, since removal of a small part of the insulation on the side does not normally jeopardize insulation between the parts, in some situations it can give rise to drawbacks.
In fact, where conductive particles of small dimensions (1-2 μm) are present in the environment, these can penetrate into the delimitation trench of the structure, in particular in the removed portion of the insulation region, and short-circuit parts biased differently, so jeopardizing operation of the device.